1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling supercharge pressure for a turbocharger.
2. Description of the Prior Art
A turbocharger is constructed in such a manner that an exhaust turbine is rotated by exhaust gas at high temperature and high pressure so that the pressure within an intake manifold increases above atmospheric pressure, as the number of rotations of a compressor in the turbocharger increases. As a result, the supply of a large quantity of intake air to the engine becomes possible by the supercharge pressure thus obtained and with the result that high torque, high output can be obtained and an improvement of the fuel consumption can be obtained as well.
Now, in a car engine having a wide range of engine speeds, it is possible to sufficiently secure the supercharge pressure in middle and high speed operating zones. In the low speed operating zone, however, as it is diffiecult to obtain sufficient exhaust pressure and the torque will tend to also be low. In this case, it is known that the supercharge pressure in the lower speed operating zone is determined by the ratio A/R of the cross-sectional area A of a scroll and the radius R from the center of the scroll. Accordingly, if the cross-sectional area can be made small in the low speed operating zone where the exhaust gas flow rate is small, the supercharge pressure can be increased by increasing the number of turbine rotations.
To this end, a turbocharger having a variable capacity in which a capacity changing means is provided for varying the the ratio A/R of the turbine, has already been proposed by the same inventors named herein (see, for instance, Japanese Patent Application SN 58-162918). In the variable capacity turbocharger, sufficient supercharge pressure can be obtained even when the turbocharger is operated in the low speed operating zone.
In the supercharge pressure control of the turbocharger described above, a provision is made for an actuator for driving the capacity changing means of the turbocharger, with the supercharge pressure being a work pressure or operation pressure which is produced downstream of a compressor and the supercharge pressure is maintained constant by controlling the duty value of an electromagnetic valve which discharges the operating pressure to the outside.
In a control characteristic of the eletromagnetic valve where the X-axis indicated duty value (see, for instance, FIG. 5), the duty value signifies the opening time of the valve per a predetermined time and when it is 100 percent it indicates that the valve is fully opened and the cross-sectional area A is made to be a minimum, in this case by means of the actuator and the capacity changing means, so as to increase the number of turbine rotations.
On the other hand, when the duty value is zero, it indicates that the electromagnetic valve is fully closed, with the result that the sectional area A is made to be a maximum and the number of turbine rotations is suppressed, thus controlling the supercharge pressure to be a constant. In the actual control, in this case, in order to overcome the deviation of control involving different dispersion factors, it is a common practice to perform a feedback control in response to an actually detected value. In this example as well, the amount of feedback correction is calculated from the deviation between the actual supercharge pressure detected by a supercharge pressure sensor and a preset supercharge pressure.
The capacity changing means of the type described above is provided for the purpose of securing sufficient supercharge pressure in the low speed and low load operating zone of the engine. In the high load operating zone, however, the number of turbine rotations becomes too high to control the supercharge pressure. Accordingly, it is a common practice to provide at the down-stream of the turbine, an exhaust bypass valve for bypassing the pressure in the turbine inlet for discharging, so as to prevent the supercharge pressure from exceeding an allowable maximum pressure. However, the use of such a plurality of control means for controlling the supercharge pressure as described above will possibly result in unexpected disadvantages as a result of interference between the two controls. Namely, the capacity changing means and the exhaust bypass valve are both controls for the supercharge pressure so that the action of one control affects the other. For instance, when the capacity changing means is progressively closed, i.e., moved in the direction of increasing the supercharge pressure from an optimum value, and the exhaust bypass valve is moved toward an open position i.e., in the direction of suppressing the supercharge pressure from the optimum value, the supercharge pressure remains at a preset value as a whole.
In this manner, in order to achieve the object of maintaining the supercharge pressure at the present value there will be numerous approaches to realize the control of the capacity changing means and the exhaust bypass valve. However, in order for the engine performance to be at its maximum, it is necessary for the capacity changing means and the exhaust bypass valve to be set at their optimum positions respectively, while maintaining the supercharge pressure at the predetermined valve.